1. Field of the Invention
The invention generally relates to WLAN (Wireless Local Area Network) receivers, and in particular to synchronization processes in such WLAN receivers.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
When operating a WLAN receiver, code synchronization is necessary because the code is the key to despreading the desired information. A good synchronization is achieved when the coded signal arriving at the receiver is accurately timed in both its code pattern position and its rate of chip generation.
Referring now to FIG. 1, the synchronization process performed in the WLAN receiver can be divided into two phases. First, a synchronization acquisition is performed in step 100 to initially synchronize the receiver with a received signal. The second part of the synchronization follows the initial acquisition since the receiver must continue to operate in such a way that it remains locked with its code reference. That is, the receiver exactly tracks in step 110 the coded incoming signal to cause its own code chip rate to match the incoming code chip rate as precisely as possible.
With respect to the synchronization algorithms used, receivers may be classified into data-aided and non data-aided receivers. The data-aided approach does not require a prior knowledge of the interference parameters but requires a training data sequence. Non data-aided (or blind) algorithms require no training data sequence but only knowledge of the desired user signal sequence and its timing.
Synchronization circuits in existing WLAN receivers still have a number of problems. One problem is that conventional circuits may be unstable in operation and sometimes work unreliably. Further, the circuits often are highly involved and therefore lead to high circuit development and manufacturing costs. Another disadvantage of existing synchronization circuits is that the phase adjustment process may be performed with insufficient digital resolution. Moreover, instability situations may occur in synchronization loops when performing the synchronization acquisition.